Process for removing ash from precipitators

ABSTRACT

A process for removing fuel ash from precipitator bottoms circulates water through the bottoms to carry off the ash, then passes the water at least in part through a hydrocyclone separation stage to produce a major overflow fraction of reduced solids content and a minor overflow fraction of increased solids content, and recirculates the water withdrawn from the bottoms less the underflow fraction back through the bottoms at a rate to maintain the solids concentration less than that at which substantial settling out of solids will occur. Preferably, the recirculation is continuous, and the underflow from said hydrocyclone separation stage is subjected to secondary stage hydrocyclone separation from which the underflow may be subjected to tertiary stage solids separation, the overflow from one or both further separation stages being recirculated.

United States Patent [191 Hirsch et a1.

[ PROCESS FOR REMOVING ASH FROM PRECIPITATORS Inventors: Ernest G.Hirsch, Foxboro; George L. Nelson, Westwood, both of Mass.

Assignee: New England Power Company,

Westboro, Mass.

Filed: July 13, 1972 App]. No.: 271,498

OTHER PUBLICATIONS The Hydrocyclone" by D. Bradley, Pergamon Press Jan.29, 1974 Ltd., 1965, pp. 41, 42, and 250.

Primary Examiner-John Adee Attorney, Agent, or Firm Edgar H. Kent [5 7]ABSTRACT A process for removing fuel ash from precipitator bottomscirculates water through the bottoms to carry off the ash, then passesthe water at least in part through a hydrocyclone separation stage toproduce a major overflow fraction of reduced solids content and a minoroverflow fraction of increased solids content, and recirculates thewater withdrawn from the bottoms less the underflow fraction backthrough the bottoms at a rate to maintain the solids concentration lessthan that at which substantial settling out of solids will occur.Preferably, the recirculation is continuous, and the underflow from saidhydrocyclone separation stage is subjected to secondary stagehydrocyclone separation from which the underflow may be subjected totertiary stage solids separation, the overflow from one or both furtherseparation stages being recir culated.

16 Claims, 5 Drawing Figures PRECIPITATOR BOT TOMS MAKE UP WATER 46 a 5sE 4 32 58 54 5o 52 TO SECONDARY OR DISPOSAL PMENIEDJAH 29 mm 3 788,469

SHEET 1 OF 3 PRECIPITATOR BOT TOMS MAKE UP WATER PMENTED K I974 3,788,469

SHEET 2 OF 3 73 TO RECIRCULATION PRIMARY UNDERFLOW SURGE TANK PROCESSFOR REMOVING ASH FROM PRECIPITATORS BACKGROUND OF THE INVENTION 1. Fieldof the Invention This invention relates to the removal of ash from fluegases, more particularly to a wet process for removing the ash fromprecipitators.

2. Description of the Prior Art For reasons of the ecology, it isimportant to separate the ash from flue gases discharged to atmospheresuch as the flue gases of oil or coal fired power plants, to preventcontamination of the atmosphere thereby. Normally, such separation iseffected by precipitating or settling the ash in electrostaticprecipitators, scrubbing towers, ash pits, and the like, hereingenerally called precipitators. The ash must in turn be removed from theseparating equipment, and this has been done by collecting the ash inwater which carries it away from the separator.

While the percentage of ash to fuel is low particularly in oil, theamount thereof separated is substantial over a period of time and mayamount to several tons a day in large power plants. The precipitatedash, if allowed to stand in the precipitator bottoms for any substantialtime, settles out on the bottoms and their sides and cools as a hardprecipitate, so hard that it is often necessary to break it off withjackhammers in order to defoul the bottom. In addition, if the ashconcentration becomes sufficiently high, settling out of the ash occursin, and fouls, the exit piping and connections from the bottoms.

The foregoing difficulties can be alleviated by flushing theash-receiving water from the bottoms at a frequency designed to preventsuch settling of ash in the bottoms and connected piping. But suchpractice requires vast quantities of water which cannot be recirculatedto any substantial extent without treating to remove the ash, since itsash concentration otherwise soon becomes too high. Suggestions ofcleaning recirculation water from precipitator bottoms with settlingtanks (see, e.g., U.S. Pat. No. 3,509,695) or centrifuges or filters(see, e.g., U.S. Pat. Nos. 2,142,406 and 3,444,668) have not beenacceptable for ash. Settling tanks involve problems of fouling similarto those encountered with the bottoms themselves; and are wasteful ofland area. Centrifuges and filters are difficult to operate effectivelyon ash at the low concentrations concerned, and are very expensive,including the need for manual supervision. Accordingly, it has been acommon practice to return the water circulated through the bottoms toocean or stream with or without lagooning, despite adverse effects ofsuch practice on the ecology.

SUMMARY OF THE INVENTION An object of this invention is to provide aprocess for the removal of ash from precipitators by circulating waterthrough their bottoms at a rate sufficient to prevent foulingaccumulation of ash in the bottoms, treating the water to remove ashtherefrom at a rate sufficient to keep the ash concentration below thatat which fouling of the bottoms and piping will occur and recirculatingthe treated water to the bottoms.

Another object is to provide such a process wherein the major part ofthe water used is recirculated and the 2 equipment required isrelatively inexpensive in capital and operation costs.

A further object is to provide such a process wherein nearly all of thewater used is recirculated, the ash separation being effected by atleast primary and secondary stage treatments, the treated water fromeach stage being recirculated.

Still another object is to provide such a process wherein valuablecomponents of the ash are segregated and recovered.

It has been discovered that hydrocyclones are peculiarly adapted for theash concentration control of the recirculated precipitator bottom watersconcerned, particularly in a primary stage, but also in a secondarystage of treatment if one is used, preferred hydrocy' clones beingsimilar in construction and operation to those used in the paperindustry for separating dirt and other foreign matter from paper-makingfiber in aqueous suspension.

In a primary stage of treatment the centrifugal force separation by thehydrocyclones is relatively low, i.e., total suspended solids removed,particularly initially after start-up with new water when the ashconcentration is extremely low. However, the ash concentration of thewater recirculated through the hydrocyclones and the bottoms, whileincreasing for a time after startup, tends to stabilize, virtuallypermanently. at or below a percentage which is low enough to preventsettling of the ash and fouling of the bottoms as previously described.In the case of oil ash this percentage may be below 3 percent by weight.The reasons for such stabilization may include a balancing in acontinuous separation operation of the amount of incoming ash particlesfrom the precipitators with the removal rate of the hydrocyclones plusan interaction between solids in solution and suspended solids.

In addition to the foregoing advantages hydrocyclones can be effectivelyoperated at very low bleed rates so that the underflow from a primarystage to a secondary treatment stage or disposal is only a smallpercentage, for example, 5 percent or less, of the total flow thereto.This in turn means that the primary stage underflow is so small and sorelatively concentrated that it can be handled easily in furtherconcentrating or disposal systems.

In a preferred practice of the invention, water is constantly circulatedto the precipitator bottoms to receive the ash therefrom and carry itoff. All or part, usually less than all, such as 30-7O percent, of theashcontaining water from the bottoms is passed in parallel through abattery of primary stage hydrocyclones operating with a low volumetricbleed rate, e.g., about 2' percent, of concentrated ash solids andwater. The overflow from the hydrocyclones is recirculated to thebottoms together with any fraction of the water from the bottoms thatwas not treated in the hydrocyclones. The underflow, containingconcentrated ash solids separated to the hydrocyclone outer wall bycentrifugal force, is passed through a battery of secondary stagehydrocyclones in parallel which may have the same or a somewhat higherbleed rate than that of the primary stage, e.g., up to about 10 percent.

The overflow from the secondary stage hydrocyclones isrecirculated tothe precipitator bottoms. The underflow from the secondary hydrocyclonesis a very small percentage of the total circulation to the bottoms(e.g., less than about 0.5 percent) and has a high concentration of ashsolids compared to the underflow from the primary stage hydrocyclones.It is therefore readily disposable with or without further treatment.Preferably, the underflow from the secondary stage hydrocyclones istreated for even further concentration of ash solids, as in one or morecentrifuges or filters, the overflow from this tertiary stage treatmentbeing recirculated to the precipitator bottoms While the underflow isdisposed of, as by land fill.

Where a tertiary stagev separation is made, the amount of make-up waterthat needs to be added to the total recirculation through theprecipitator bottoms is so relatively minute that the system is nearly atotal recirculation system, less removed solids. However, tertiary stagetreatment, and even a secondary stage treatment, can be omitted. Theunderflow from the secondary stage is so relatively small compared tothe total recirculation flow that, in many cases, it may be disposed ofas such without difficulty. Particularly with small plants, theunderflow from the primary stage is also sufficiently low to enablesatisfactory disposal without further treatment.

The rate of recirculation to the precipitator bottoms is preferably suchthat the concentration of solids received by the water from theprecipitators, at each pass, is low, for example less than 0.1 percentby weight. At such low concentrations the hydrocyclones have arelatively low solids content in the bleed flow butas the solidsconcentration increases the solids removal in the hydrocyclonesincreases and the solids concentration tends to stabilize below thedesired maximum.

In an alternative, although not preferred, practice of the process,water may be intermittently circulated to the precipitator bottoms. Forexample, the bottoms may be filled with water and amptied and flushedsuccessively after each has reached a predetermined maximumconcentration, below the dangerous, settling-out concentration. In sucha system the bottoms may be equipped with mechanical agitators tomitigate solids settling, and the agitators may be equipped to signalfor emptying and flushing when the torsional resistance of the agitatorreaches a predetermined maximum. Or the bottoms may be emptied andflushed in a predetermined timed sequence.

The underflow from any stage may be treated for the recovery of valuablecomponents of the ash, as by chemical precipitation or agglomeration.However, it has been noted that certain valuable components, such asvanadium pentoxide and nickel salts in the case of certain types of oilash, tend to concentrate in the recirculating water, particularly ifthese components are to some extent water-soluble. Where the quantity ofsuch valuable components warrants, our process includes continuously orintermittently diverting recirculation flow from the primary stage fortreatment for the recovery of desired valuable components as by chemicalprecipitation.

BRIEF DESCRIPTION OF THE DRAWINGS In the drawings:

FIG. 1 is a flow sheet of the process in a primary stage treatment, withcertain options indicated in dotted lines;

FIG. 2 is a diagrammatic illustration of a system of recirculating waterto the precipitator bottoms in the flow sheet of FIG. 1;

FIG. 3 is a flow sheet showing a secondary stage treatment that may beused with the primary stage of FIG. 1, and also a tertiary stagetreatment;

FIG. 4 is a diagrammatic illustration of a modified system forrecirculating water to the precipitator bottoms; and

FIG. 5 is a graph showing percent solids in the feed to a primary stageof the process according to FIG. 1 over a 100 hour period of operationfrom start-up.

DESCRIPTION OF THE PREFERRED EMBODIMENTS Referring first to the primarystage flow sheet of FIG. 1, the precipitator bottoms are thereinindicated by a box so labeled. Water is circulated out of all thebottoms to a header pipe 10 which discharges into a surge tank 12. Pipe14 from a bottom outlet of tank 12 goes to the suction side of pump 16which discharges through pipeline 18 which goes to the inlet side of theprecipitator bottoms through a valve 20. A branch pipe 22 leads frompipe 18, between valve 20 and pump 16, to a three-way variable flowdistributing valve 24 which has two output pipes 26 and 28. Branch pipesfrom pipe 26 are connected to the inlets of a battery of hydrocyclonesdesignated generally 30, while pipe 28 is a return line to tank 12.

The overflow or accepts outlets from the hydrocyclones are connected toa header pipe 32 which is shown in full lines as returning the overflowto tank 12. The underflow or rejects outlets from the hydrocyclonesdischarge to a tank or trough 34'and pass therefrom through pipeline 36to a pump 38 which discharges them through a pipeline to secondary stageseparation or disposal as indicated by its label To Secondary orDisposal." The equipment indicated by full lines in FIG. l is completedby an input pipe to tank 12 for make-up water, as indicated by itslabel.

A usual operation of the system shown in full lines in FIG. I is toadjust valves 20 and 24 so'that part of the flow from pump 16 in pipe18, such as 30 to percent thereof, is diverted into pipe 22 and directedby valve 24 into pipe 26, with pipe 28 closed by valve 24. Thisarrangement works well when the rate of ash deposit from theprecipitator bottoms into the circulated water, and the watercirculation rate through the bottoms, are such that the solids depositedin the water in a single pass through the bottoms produces a low percentso] ids, such as a small fraction of 1 percent. As operation continues,the solids concentration in the recirculated water entering tank 12 frompipe 10 increases but is diluted by the hydrocyclone overflow from pipe32 at lower solids concentration. As the solids concentration in thefeed to the hydrocyclones increases, the amount of solids separatedthereby to tank 34, pipe 36 and pump 38 also increases, until it tendsto stabilize around an average value. It may be desirable from time totime to adjust the pH of the circulating flow by adding a chemical suchas lime. Such chemical addition may be made in tank 12 and to this endthere is shown in dotted lines a chemical supply tank 40 with outputpipeline 42 to a pump 44 and thence into tank 12. Pump 44 may becontrolled automatically or manually under the supervision of a pH meterin tank 12. Valve 24 may be adjusted to return to tank 12 via pipeline28 the fraction of the output of pump 16 which goes to pipe 22. Thispermits isolation from the circulation system of the hydrocycloneswithout terminating or changing the circulation to the precipitatorbottoms or through tank 12, as may be desirable from time to time.

Treating only part of the recirculation flow has certain advantages. Itenables the use of proportionally more water in the total circulationthan is needed for recirculation through the precipitator bottoms, whichdesirably lowers the solids concentration fluctuations. Also, fewerhydrocyclones are required than would be needed to treat all thecirculation flow at the desired operating feed rate. However, where theash content of the gas treated in the precipitators is high (it isusually condiderably higher where coal rather than oil is the fuel), itmay be desirable to pass the entire recirculation flow to theprecipitator bottoms through hydrocyclones. This may be done on'aconstant or intermittent basis.

FIG. 1 indicates by dotted lines an arrangement permitting treatment ofthe entire recirculation flow in hydrocyclones. A pipeline 46 leads fromthe hydrocyclone overflow header line 32 to pipe 18. A valve 48 in line32 may be closed to prevent flow through line 32 beyond it and to divertall hydrocyclone overflow through pipeline 46 to pipeline l8 and thencethrough the precipitator bottoms. Valve may be closed so that all flowfrom pump 16 is diverted through pipe 22, valve 24 and pipeline 26 tothe hydrocyclone inlets. Additional hydrocyclones can be provided asneeded to handle the added flow.

If the flow sheet is designed for constant treatment of allrecirculation flow in hydrocyclones, pipe 26 is connected directly topump 16, line 46 is connected directly to the precipitator bottoms, andpipes 18 and 22, valves 20 and 24 and the portion of header pipeline 32beyond pipe 46 with valve 48 are omitted.

If the hydrocyclone underflow output of pump 38 goes to a secondaryseparation stage as is preferred, the effluent from that stage may bereturned to the recirculation flow by a pipeline indicated by dottedline 50, which discharges into tank 12. If the underflow from thesecondary stage treatment goes to a tertiary stage separation treatmentas shown in FIG. 3, the overflow from that stage may be returned to therecirculation flow via a pipeline indicated by dotted line 52,discharging into tank 12. Dotted extension 54 of pipeline 32 havingvalve 56 permits overflow from the hydrocyclones to be diverted fortreatment for the recovery of one or more components of the containedash, which are of sufficient value and are present in sufficientquantity to warrant such treatment. Valve 56 may be adjusted to divert asmall fraction of the flow in pipeline 32 to pipeline 54 on a constantbasis or valve 56 may be opened and closedintermittently forintermittent diversion of overflow to pipeline 54. A valve 58 in line 32may be closed so that all hydrocyclone overflow may be intermittentlydiverted to pipeline 54.

FIG. 2 shows an arrangement for recirculating the flow in ash-receivingexposure to the ash precipitated by the precipitators. The precipitatorbottoms, partially indicated at 60, which may have the form of invertedtruncated hollow pyramids or cones, open directly ceives the ash as afall-out from the bottoms. The inclined side walls of the bottoms 60 arepreferably shaken or vibrated by external mechanical means (notindicated in the drawing) to insure the sliding of ash deposited thereoninto the flowing stream in pipe 62. In the flow sheet of FIG. 1, pipe 62is supplied by pipeline l8 and discharges to pipeline M).

In the arrangement of FIG. 2 the ash, received in rapidly moving waterflow, has no opportunity to settle, and is kept from settling thereafterby the rapid recirculation flow conditions of the process.

In FIG. 3, the hydrocyclone underflow from the primary stage is suppliedby pump 38 of FIG. 1 and its output pipeline to an inlet manifold pipe70, having branch connections to the inlets of a battery of secondaryhydrocyclones indicated generally at 72. Tank 34 in FIG. 1 functions asa surge tank for pump 38 and pipe of FIG. 3 to insure constant regulatedfeed to the hydrocyclones 72 despite'fluctuations in the underflow fromthe primary stage, or to provide for intermittent operation ofhydrocyclones 72' if the underflow from the primary stage isinsufficient to feed hydrocyclones 72 constantly at the desiredoperating rate.

The overflow from hydrocyclones 72 discharges to header pipe 73 whichreturns the overflow either to the recirculation system of FIG. 1through connected pipe 50 of FIG. 1, or to tank 34 for recirculationthrough hydrocyclones 72 by a connection (not shown) from pipe 50 totank 34. The underflow from hydrocyclones 72 discharges into tank ortrough 74 from which drain pipe 76 goes to pump 78 having output pipe80. In this stage, the hydrocyclone underflow is of such relativelysmall volume and high concentration of solids (e.g., 10 to 40 percent orhigher) that it may be in the desired condition for disposal and, if so,is sent directly thereto (e.g., a truck or barge) from pipe 80 via aconnecting pipe indicated by dotted line 82.

However, the secondary stage underflow volume and concentration are alsosuch that it can be dewatered effectively and relatively inexpensivelyto a substantially dry state, which is frequently desirable. For thispurpose, in the flow sheet of FIG. 3, a pipe 84 connects pipe 80 to thefeed pipe of a continuous centrifuge 86. The effluent or overflow fromthis tertiary stage centrifugal separator 86 may be returned to therecirculation system of FIG. 1 via pipeline 88 which is connected topipe 52 of FIG. 1, or it may be returned to tank 34 or tank 74 byconnections (not shown) from pipeline 88. The solids output or underflowfrom the centrifuge goes to disposal as indicated by the line so marked,for example, by conveyor to a truck or storage bin. Trough 74 can beused as a surge tank if the secondary underflow is insufficient topermit continuous operation of the centrifuge.

FIG. 4 shows another arrangement of precipitator bottoms. In thisarrangement, the bottoms are panshaped, extend beneath the entireprecipitator section, contain water and have no inclined sides as inFIG. 2 to impede the free fall of the precipitated ash into the water.

In FIG. 4, the precipitator bottoms 90 may be filled to a desired levelup to.overflow pipes 92 via input header pipeline 94 and branchconnections 96 with valves 98. The bottoms may be dumped or drainedthrough outlet pipes 100 having valves 102 and connected to headerpipeline 104. Overflow pipes 92 are also connected to a header pipeline104 which goes to a surge tank such as tank 12 of FIG. 1, which has morethan sufficient capacity to hold the liquid content of one bottom 90.The bottoms 90 are provided with agitators 106 having external motorsindicated by blocks M.

The arrangement of FIG. 4 may be utilized with intermittent fill anddrain of each bottom successively. To dump and flush a bottom, its valve102 is opened so that the contents drain through pipes 100 and 104 tothe surge tank. Its valve 98 is then opened, the bottom being allowed toflush so that the water flows out line 100 and through pipeline 104 tothe surge tank as fast as it is fed into the bottom via its inlet pipe96. The rapid flow of water through the bottom flushes out solids thatmay have settled out thereon during the time between fill and dump, andalso receives and carries away the ash falling to the bottom whileflushing cons tin ues. When flushing is completed the till cycle isstarted. To fill a bottom, its valve 102 is closed and its valve 98 isopened until the water level in the bottom reaches a predeterminedheight below its overflow pipe 92, as may be signaled by a float valveor the like,

, whereupon valve 98 is closed and remains closed together with valve102 until the dump and flush cycle is repeated.

The arrangement of FIG. 4 may be utilized in the flow sheet of FIG. 1,in which case pipeline 104 is connected to pipeline of FIG. 1 andpipeline 94 is connected to pipeline 18 of FIG. 1 (or directly topipeline 46 if the system is designed for circulating all the output ofpump 16 through the hydrocyclones on a continuous basis). The operationof the process is the same as shown in the flow sheet of FIG. 1 and alsoin the flow sheet of FIG. 3 if secondary separation is used, with orwithout a tertiary stage, except that the system works on the contentsof one bottom at a time. Thus, part only of the output of pump 16 may becirculated to hydrocyclones 30, with the remainder recirculated to thebottom being flushed and the hydrocyclone effluent being returned totank 12. Alternatively, all of the output of pump 16 may go tohydrocyclones 30, with their overflow being recirculated to flush thebottom. The extra capacity of tank 12 allows continuous operation of thesystem at substantially constant flows.

The precipitator bottoms of FIG. 4 may be filled, dumped and flushedsuccessively on a uniform time basis in which the time between fill anddump of each bottom is sufficiently short so that the accumulation ofsolids inthe water therein does not build up to a concentration suchthat significant settling out of solids will occur to foul the bottomsor the rest of the system, while the flushing time is sufficient toremove settled solids from the bottoms and to permit the hydrocyclonesto reduce the solids percentage to a desired low level.

The motors M of stirrers 106 may be equipped with torsional resistancemeasuring devices to signal if the solid contents of the water in anybottom is approaching an undesirably high percentage. In the case ofsuch signal, the bottom concerned may be dumped and flushed out of turnif not next in order, with dumping of the next bottom in the timesuccession delayed for the requisite time, or occurring simultaneouslywith dumping of the signaling bottom if the capacity of the treatingsystem is adcquateto process the contents of two bottoms at the sametime. If there is time delay between filling of one bottom and dumpingof the next succeeding bottom in the time succession, the remainingcontents of tank 12 may be recirculated back to it via pump 16, pipes 18and 22, valve 24 and pipe 28, or, if these connections are not present,through a by-pass line connecting pipeline 18 or 46 to line 10.

It will be apparent that the recirculation system of FIG. 4 can beoperated on a continuous, simultaneous flushing basis for all bottoms,and such is the preferred operation. In such case, the operation wouldbe similar to that of FIG. 2.

Preferred hydrocyclones are of the type used in the paper industry forcleaning paper pulp, such as the hydrocyclones sold by Bird 7 MachineCompany, Inc. under the trademark Cyclean, for example, of 4 inchdiameter having a length to diameter ratio in the order of 6:1 to 10:1and operating at a pressure drop of about 40 psi. This type ofhydrocyclone utilizes the high centrifugal force which is generated bythe small body diameter. The narrow cone angle characteristic of a pulphydrocyclone enables the unit to separate and concentrate the low micronsized particles found in fuel ash, and to operate effectively with a lowbleed rate of the underflow in the primary stage as is desirable.

FIG. 5 shows percent solids of the primary feed in a pilot operationover a I00 hour period removing oil ash from eight electrostaticprecipitator bottoms. The bottoms and water circulation to and from themwere like FIG. 2. The primary flow sheet was like FIG. 1. Half of thepump output of 300 g.p.m. from the surge tank was recirculated to thebottoms and back to the surge tank, the other half being fed in parallelto four 4-inch diameter hydrocyclones of the type mentioned above. Theunderflow from the primary hydrocyclones at 3 g.p.m. was fed to a singlesecondary hydrocyclone, but since its volume was insufficient forconstant feed to the secondary hydrocyclone, it was stored in a tank andfed intermittently to that hydrocyclone, with the overflow beingreturned to the primary surge tank. The eight precipitator bottoms hadan average total ash output of about 2.4 pounds per hour.

The system was started with mixed fresh and salt water. After 1 hour thesolids concentration was less than 0.1 percent. By 50 hours it hadincreased to just over 1 percent. It then increased rapidly to near 2percent and then fell off, also rapidly, to near 1 percent, leveling offat an average between 1 percent and 1.5 percent; The fluctuations inpercent solids were probably due largely to fluctuations in ash outputfrom the precipitator bottoms. The underflow from the secondaryhydrocyclone was at a rate of about 2 percent of the feed thereto andhad a solids concentration averaging slightly below 10 percent.

When the solids content leveled off, the hydrocyclone system reached astate of equilibrium, namely, that the system rejected per unit of timeapproximately the same amount of solids that entered the water from theprecipitators.

It will be understood that in large installations with manyprecipitators, such as a 1000 MW or more power plant, severalinstallations according to the flow sheet of FIG. 1 will usually beneeded to operate in parallel on different sets or systems ofprecipitators. Otherwise, the equipment required to handle the flowvolume for processing all the ash would be excessively large. In such apower plant currently being converted to the process, separate suchinstallations are being made for processing the ash from theprecipitator systems of two oil-fired power units, one a 650 MW unit ofwhich the design rate of the precipitator system is about 93 pounds ofash per hour and the other a 450 MW unit which has a precipitator systemdesign rate of about 80 pounds of ash per hour. The circulating flow tothe 650 'MW unit precipitator system in an arrangement like FIG. 2 willbe about 2,400 gallons per minute or about 144,000 gallons per hour, andsuch flow to the 450 MW unit precipitator system will be about 2,000gallons per minute or 120,000 gallons per hour with a total flow forboth at 264,000 GPH. It will be noted that the circulation rate forthese two precipitator systems is about 1,500 or more gallons per hourper pound of ash from the system, and it is preferred that thecirculation rate be at least 1,000 gallons per pound per hour.

On the other hand, the underflow from the primary stage hydrocyclones isso relatively small that it can be processed in one secondaryhydrocyclone stage as in FIG. 2, having a single underflow output butparallel return lines to the two primary systems for the overflow in theinstance mentioned above. In this system the underflow from the primaryunits would be about 3,600 GPH, and from the secondary units only about90 GPl-l, which represents a flow concentration of about 3,000 to 1.

We claim:

1. A process for removing fuel ash from a precipitator system whichcomprises the steps of:

collecting the ash from the precipitator system in water exposedthereto;

withdrawing ash-containing water from said system and subjecting atleast part thereof to centrifugal separation in a hydrocyclone separatorstage to provide a major overflow fraction of reduced solids content anda minor underflow fraction of increased solids content; recirculatingsaid withdrawn water less said underflow fraction to receive ash fromsaid system; and

repeating said steps with said recirculated water at a frequency and atsuch volumetric flow rates as to maintain the solids concentration inthe water exposed to said precipitator system below a concentration atwhich substantial settling-out of suspended solids will occur.

2. A process according to claim 1 wherein said withdrawal andrecirculation steps are continuous and at the same volumetric flow rate.

3. A process according to claim 1 wherein said withdrawal andrecirculation steps are applied successively and in multiple todifferent precipitator bottoms of said system.

4. A process according toclaim 1 wherein about 30 to percent of saidwithdrawn ash-containing water is subjected to said centrifugalseparation.

5. A process according to claim 2 wherein said ash from said system iscollected in a continuously flowing stream of said water exposedthereto.

6. A process according to claim 1 wherein said minor underflow fractionis of the order of 5 percent or less of the feed to said primaryhydrocyclone separator stage.

7. A process according to claim 1 which includes the further steps of:

subjecting said minor underflow fraction to centrifugal separation in asecondary hydrocyclone separator stage to provide a major overflowfraction of reduced solids content and a minor underflow fraction ofincreased solids content; and

recirculating said major overflow fraction of said secondaryhydrocyclone separator stage.

8. A process according to claim 7 wherein said minor underflow fractionof said secondary hydrocyclone stage is of the order of 2 to 10 percentof the feed to said last named stage.

9. A process according to claim 7 wherein said minor underflow fractionof said secondary hydrocyclone stage is less than about 0.5 percent ofsaid recirculated withdrawn water.

10. A process according to claim 7 which includes the further step ofsubjecting said minor underflow fraction of said secondary hydrocycloneseparation stage to a tertiary stage solids separation treatment toprovide a major overflow fraction of reduced solids content and a minorunderflow fraction of increased solids content.

11. A process according to claim 10 wherein said major overflow fractionof said tertiary stage treatment is recirculated.

12. A process according to claim 10 wherein said tertiary stagetreatment is centrifuging.

13. A process according to claim 10 wherein said tertiary stagetreatment is filtering.

14. A process according to claim 1 which includes the further step ofsegregating a part of said withdrawn recirculated water for treatmentfor recovery of solids therefrom.

15. A process according to claim 1 which includes the further step ofadding chemical to said withdrawn recirculated water.

16. A process according to claim 2 wherein said volumetric flow rate isat least 1,000 gallons per hour per pound of ash discharged from saidprecipitator system.

UNITED STATES PATENT OFFICE CERTIFICATE OF CORRECTION Patent No. 3,788l69 Dated January 29, 197 4 Inventor(s) I Ernest G. Hirsch et 31.

It is certified that error appears in the above-identified patent andthat said Letters Patent are hereby corrected as shown below:

Abstract, line 6, change "overflow" to ---underflow--.

Col. 3, line 36, change "amptied" to -emptied--.

Signed and sealed this 21st day of May 1974.

(SEAL) Attest:

EDWARD M.FLETCHER,JR. v C. MARSHALL DANN Attesting Officer Commissionerof Patents

2. A process according to claim 1 wherein said withdrawal andrecirculation steps are continuous and at the same volumetric flow rate.3. A process according to claim 1 wherein said withdrawal andrecirculation steps are applied successively and in multiple todifferent precipitator bottoms of said system.
 4. A process according toclaim 1 wherein about 30 to 70 percent of said withdrawn ash-containingwater is subjected to said centrifugal separation.
 5. A processaccording to claim 2 wherein said ash from said system is collected in acontinuously flowing Stream of said water exposed thereto.
 6. A processaccording to claim 1 wherein said minor underflow fraction is of theorder of 5 percent or less of the feed to said primary hydrocycloneseparator stage.
 7. A process according to claim 1 which includes thefurther steps of: subjecting said minor underflow fraction tocentrifugal separation in a secondary hydrocyclone separator stage toprovide a major overflow fraction of reduced solids content and a minorunderflow fraction of increased solids content; and recirculating saidmajor overflow fraction of said secondary hydrocyclone separator stage.8. A process according to claim 7 wherein said minor underflow fractionof said secondary hydrocyclone stage is of the order of 2 to 10 percentof the feed to said last named stage.
 9. A process according to claim 7wherein said minor underflow fraction of said secondary hydrocyclonestage is less than about 0.5 percent of said recirculated withdrawnwater.
 10. A process according to claim 7 which includes the furtherstep of subjecting said minor underflow fraction of said secondaryhydrocyclone separation stage to a tertiary stage solids separationtreatment to provide a major overflow fraction of reduced solids contentand a minor underflow fraction of increased solids content.
 11. Aprocess according to claim 10 wherein said major overflow fraction ofsaid tertiary stage treatment is recirculated.
 12. A process accordingto claim 10 wherein said tertiary stage treatment is centrifuging.
 13. Aprocess according to claim 10 wherein said tertiary stage treatment isfiltering.
 14. A process according to claim 1 which includes the furtherstep of segregating a part of said withdrawn recirculated water fortreatment for recovery of solids therefrom.
 15. A process according toclaim 1 which includes the further step of adding chemical to saidwithdrawn recirculated water.
 16. A process according to claim 2 whereinsaid volumetric flow rate is at least 1,000 gallons per hour per poundof ash discharged from said precipitator system.